bhyve: add helper to read PCI IDs from bhyve config

Changing the PCI IDs is valuable in some situations. The Intel GOP
driver requires that some PCI IDs of the LPC bridge are aligned with the
physical values of the host LPC bridge. Another use case are oracles
virtio driver. They require different subvendor ID than the default one.
For that reason, create a helper which makes it easy to read PCI IDs
from bhyve config. Additionally, this helper ensures that all emulation
devices are using the same config keys.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D38402

(cherry picked from commit ffaed739a80319a56d0b1dfd6cedeb3d194ff42c)